The present invention relates to a damping force control type hydraulic shock absorber attached to a suspension system of a vehicle, for example, an automobile.
Hydraulic shock absorbers attached to suspension systems of automobiles or other vehicles include damping force control type hydraulic shock absorbers that are designed so that damping force characteristics can be appropriately controlled in accordance with road surface conditions, vehicle running conditions, etc. with a view to improving ride quality and steering stability.
In general, a damping force control type hydraulic shock absorber includes a cylinder having a hydraulic fluid sealed therein. A piston is slidably fitted in the cylinder to divide the inside of the cylinder into two chambers. The piston is connected with a piston rod to form a piston assembly. The piston assembly is provided with a main hydraulic fluid passage and a bypass passage, which provide communication between the two chambers in the cylinder. The main hydraulic fluid passage is provided with a damping force generating mechanism including an orifice, a disk valve, etc. The bypass passage is provided with a damping force control valve for varying the passage area of the bypass passage to control damping force.
When the bypass passage is opened through the damping force control valve, the flow resistance to the hydraulic fluid flowing between the two chambers in the cylinder is reduced, thereby reducing damping force. When the bypass passage is closed, the flow resistance between the two chambers is increased, thereby increasing damping force. Thus, damping force characteristics can be appropriately controlled by opening or closing the damping force control valve.
The above-described damping force control type hydraulic shock absorber, in which damping force is controlled by varying only the passage area of the bypass passage, has the following disadvantage. Although the damping force characteristics can be changed to a considerable extent in a low piston speed region because damping force depends on the restricting action of the orifice in the hydraulic fluid passage, the damping force characteristics cannot greatly be changed in intermediate and high piston speed regions because in these regions damping force depends on the degree of opening of the damping force generating mechanism (disk valve, etc.) in the main hydraulic fluid passage.
Under these circumstances, Japanese Patent Application Unexamined Publication (KOKAI) No. Hei 7-332425 (corresponding U.S. Pat. No. 5,655,633; corresponding Offenlegungsschrift DE 195 18 560 A1), for example, discloses a damping force control type hydraulic shock absorber in which a pilot-operated damping valve control valve is provided as a damping force generating mechanism in a main hydraulic fluid passage common to the extension and compression sides. In the pilot-operated damping valve control valve, a back-pressure chamber (pilot chamber) is formed at the back of a disk valve. The back-pressure chamber is communicated with a cylinder chamber at the upstream side of the disk valve through a fixed orifice and also communicated with a cylinder chamber at the downstream side of the disk valve through a variable orifice (pilot control valve).
According to the above damping force control type hydraulic shock absorber, the area of the communicating passage between the two chambers in the cylinder is controlled directly by opening or closing the variable orifice. Moreover, the valve opening pressure of the disk valve can be varied by changing the pressure in the back-pressure chamber by the pressure loss in the variable orifice. Thus, it is possible to control orifice characteristics (in which damping force is approximately proportional to the square of the piston speed) and valve characteristics (in which damping force is approximately proportional to the piston speed) and hence possible to widen the control range of damping force characteristics.
However, the above-described conventional damping force control type hydraulic shock absorber having a pilot-operated damping force control valve suffers from the following problem. The pilot-operated damping force control valve is a pressure control valve and generates substantially constant damping force irrespective of the piston speed. This is convenient for controlling the vibration of a sprung member. However, damping force is likely to become insufficient for the vibration of an unsprung member. In addition, because the damping force characteristics change rapidly when the valve is opened, vibration and noise are likely to be generated. Further, because a disk valve is used as the pilot-operated damping force control valve, the set load and the spring constant vary to a considerable extent, and the arrangement is disadvantageous in terms of durability. It should be noted that the damping force characteristics can be controlled to a certain extent by disposing a sub-valve (disk valve) in series to the pilot-operated damping force control valve. In such a case, however, it becomes difficult to set the xe2x80x9csoftxe2x80x9d-characteristic damping force at a sufficiently small value because the damping force of the sub-valve is added to that of the pilot-operated damping force control valve.
The present invention was made in view of the above-described circumstances.
Accordingly, an object of the present invention is to provide a damping force control type hydraulic shock absorber having a pilot-operated damping force control valve, which is capable of obtaining ideal damping force characteristics.
The present invention provides a damping force control type hydraulic shock absorber including a cylinder having a hydraulic fluid sealed therein. A piston is slidably fitted in the cylinder. A piston rod is connected at one end thereof to the piston. The other end of the piston rod extends to the outside of the cylinder. A hydraulic fluid passage allows the hydraulic fluid to flow therethrough in response to the sliding movement of the piston. A pilot-operated damping force control valve generates damping force by controlling the flow of the hydraulic fluid in the hydraulic fluid passage. The pilot-operated damping force control valve includes a plurality of main damping valves different from each other in valve opening characteristics and a single pilot control valve for controlling the pilot pressure of the plurality of main damping valves.
With this arrangement, the plurality of main damping valves constituting the pilot-operated damping force control valve are sequentially opened or closed, thereby allowing damping force to be controlled stepwisely.
The above and other objects, features and advantages of the present invention will become more apparent from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings.